Conventionally, refining structure has been studied as a method for improving the mechanical properties of cold-rolled steel sheets.
Patent Document 1 indicated below discloses a cold-rolled steel sheet having a structure including a low-temperature transformation phase consisting of one or more of ferrite, martensite, bainite and retained γ (retained austenite), in which the volume fraction of the low-temperature transformation phase being 10 to 50% and an average grain diameter of the low-temperature transformation phase is at most 2 μm.
Patent Document 2 indicates a method in which a cold-rolled steel sheet is manufactured using a hot-rolled steel sheet manufactured by hot rolling followed by cooling in a short length of time after the hot rolling. For example, Patent Document 2 discloses that a hot-rolled steel sheet having a microstructure containing ferrite having a small average grain diameter as a main phase is manufactured by carrying out cooling to at most 720° C. at a cooling rate of at least 400° C./sec within 0.4 seconds after hot rolling and the hot-rolled steel sheet is subjected to usual cold rolling and annealing.